In a particular screening system for wood chips, comprising a gyratory screen followed by a disk screen, the disk screen is typically operated at a fixed speed. The fixed speed usually is selected to achieve a particular overthick removal efficiency (ORE). There is a practical maximum limit, however, to the overthick removal efficiency, since as ORE is increased, the carry-over of acceptable size wood chips (ACO) off the end of the disk screen also increases, which is undesirable. It is the combination of these two operating characteristics, overthick removal efficiency (ORE) and accepts carry-over (ACO) which determine the overall performance rating of a screening system. As indicated above, while a high ORE is desirable, further increases in ORE beyond a certain point will actually reduce the overall performance of the screening system, due to a more than offsetting increase in ACO. The ACO should be as low as possible. Typically, the disk screen speed is selected only after a period of experimentation with the system and the apparatus is then run at that particular speed thereafter in its operating setting.
In typical operation of the above-described combined screening system, a varying amount of material over time is provided to the input end of the disk screen from the gyratory screen. This changing quantity of mass input material results occasionally in large quantities of material being passed over the disk screen and sent to a follow-on portion of the system, a chip slicer. Large quantities of material at the chip slicer will result in the plugging or breakdown of the slicer, which in turn causes a shutdown of the entire screening system, a very undesirable result.
While it is understood that this mass flow problem can be corrected by reducing the speed of the disk screen to the extent that the slicer is never overtaxed, such a solution will reduce the ORE of the system not only when the mass flow rate is high due to a high feed rate, but also during other feed rates, including a feed rate which would result in a mass flow rate off the end of the disk screen otherwise acceptable to the slicer. While the solution to such a problem would appear to be a variable speed control for the disk screen, such a possibility has not been implemented in practice, because it was not heretofore considered to be feasible to have a feedback control dependent on the mass flow rate of material off the disk screen. For instance, it is desirable that the mass flow rate off the disk screen be relatively uniform. This requires monitoring the flow rate such that as the flow rate begins to increase, feedback control would slow the disk screen down to handle the increased flow and vice versa. However, to date there has been no practical, reliable way to monitor the mass flow rate off the disk screen. Hence, combined gyratory and disk screens have continued to use a fixed disk screen speed even though this does have undesirable consequences relative to overall system operation and efficiency.